1. Field of the Invention
This invention relates to a further development of the production of surface-active alkyl glycosides by the so-called direct synthesis method. In this process, the alkyl glycosides, i.e. acetals of sugars and monofunctional alcohols, are prepared by direct acid-catalyzed reaction of the alcohols with the sugars with elimination of water.
2. Statement of Related Art
More particularly, the present invention relates to a further development of a process of the type mentioned which is described, for example, in International patent application W090/03977 ("A Process for the Direct Production of Alkyl Glycosides"). In this document, the term alkyl glycosides is explained both in its broader form and, more particularly, in the narrower form used for practical purposes; those explanations also apply to the teaching of the present invention. The sugar components, hereinafter referred to as glycoses, comprise aldoses and also ketoses in the broadest sense. Aldoses are preferably used by virtue of their better reactivity. Glucose is particularly suitable by virtue of its ready accessibility and availability in industrial quantities, so that the invention is particularly concerned with the production of surface-active alkyl glucosides. The alkyl radical attached to the glucose by the acetal group is derived from monofunctional, preferably relatively long-chain alcohols, particular significance being attributed to corresponding alkyl radicals containing 8 to 20 carbon atoms. Particularly suitable alkyl radicals are derived from alcohols which have been obtained from natural materials, for example fats and/or oils of natural origin, although the invention is by no means limited to such alcohols. The term alkyl glycosides in this context encompasses compounds of the type mentioned in which the alkyl radicals are attached in acetal form to mononuclear and/or to polynuclear sugar residues.
Alkyl glycosides of the type mentioned, their production and their use, particularly as surface-active compounds, are the subject of numerous prior publications which are described in detail in applicants' above-cited earlier International application W090/03977. This earlier international application also relates to a process for the direct production of alkyl glycosides by acetalization of higher aliphatic primary alcohols with glycoses, particularly glucose, in the presence of an acidic catalyst, rapid removal of the water of reaction, neutralization of the catalyst with a base, removal of the alcohol excess by distillation and conversion of the reaction product into an aqueous paste and bleaching of this paste, the aliphatic alcohol being used in an excess to the glycose and the formation and removal of the water of reaction taking place in vacuo and reaction temperatures above 80.degree. C. being applied. The known process is essentially characterized in that a) mixtures of aliphatic primary alcohol, glycose and acidic catalyst are prepared and reacted at elevated temperature, either (i) part of the alcohol being initially introduced with the catalyst, the mixture being heated and a heated suspension of the glycose in the remaining quantity of alcohol being added continuously or in portions to the alcohol/catalyst mixture and the water of reaction formed being distilled off in vacuo, or (ii) a mixture of the entire alcohol and the glycose being initially introduced, heated and the acidic catalyst being added to the heated mixture, a vacuum subsequently being applied and the mixture being further heated until the reaction begins and the water of reaction being distilled off, b) the mixing ratios are selected so that the molar ratio of glycose to aliphatic alcohol is from 1:2 to 1:10 and preferably from 1:3 to 1:6, c) the reaction mixture is kept at that temperature and under that reduced pressure, preferably while mixing, until the water of reaction has been completely removed, d) the reaction mixture is subsequently cooled to approximately 90.degree. C., after which an organic or inorganic basic alkali, alkaline-earth or aluminium or alkali/aluminium compound is added in such quantities that, over and above the neutralization of the acidic catalyst, a pH value of at least 8 and preferably in the range from 8 to 10 is adjusted and normal pressure is preferably only established thereafter, e) the excess alcohol is distilled off from the alkaline mixture in vacuo, preferably without preliminary filtration, to a value below 5% by weight of the reaction product by any of the methods known per se which do not damage the reaction product and f) the mixture is subsequently cooled to approximately 105.degree. C. and a 30 to 60% paste is produced by addition of water and is stirred for about 0.1 to 5 hours at approximately 80.degree. C. by the addition, preferably in portions, of active oxygen compounds, preferably hydrogen peroxide, measures optionally being taken by addition of alkali, preferably sodium hydroxide, to ensure that the pH value remains at 8 to 10 during this bleaching process. Preferred higher aliphatic primary alcohols for the production of the alkyl glycosides contain 8 to 20 carbon atoms and, more particularly, 12 to 18 carbon atoms.
The central reaction of the direct synthesis is the acid-catalyzed acetalization of the sugars used, more particularly glucose, with the monofunctional alcohols used in excess. To produce high-quality products on the one hand having the desired predetermined constitution and, on the other hand, the desired color and color stability, it has been found that the performance of this crucial step of the direct synthesis in practice is of very considerable importance. The above-cited International application WO90/03977 mentions several possibilities in this regard. The critical step a) of the process defined in the foregoing can be carried out in two ways, namely (a, i) and (a, ii). The first procedure referred to herein has the advantage that the water of reaction formed during the acetalization and/or the water introduced through the sugar reactants, for example glucose monohydrate, fed in can be rapidly and continuously removed from the reaction mixture. This is achieved by carrying out the acetalization under a low vacuum of, for example, about 10 to 50 mbar, so that the water of reaction can be removed via the vapor phase in view of the high reaction temperatures applied, namely above 100.degree. C. and preferably of the order of 120.degree. C. According to the earlier application, this continuous addition of the sugar reactants during the reaction is preferred. It is said to be best to select the addition rate so that a substantially clear phase is permanently present in the reactor, in other words the quantity of unreacted glycose in the reaction mixture is kept to a minimum. Another important aspect of this known process is that the mixture is preferably mixed and heated continuously during the reaction with only a slight temperature difference between the reactor wall and the reaction mixture, which is regarded as essential for avoiding overheating which would lead, for example, to unwanted deteriorations in the color of the reaction product. In the further description of this known process, it is stated how, for example, reaction temperatures of around 120.degree. C. can be established in the reactor and maintained without reactor wall temperatures of more than about 125.degree. C. having to be used.